Systems for separation of air by selective sorption to recover oxygen and/or nitrogen are well known in the art. Typically, most of these systems employ zeolite molecular sieve adsorbent for selective retention of nitrogen while recovering an oxygen-enriched product stream. Periodically, as the sorbent bed reaches a designed level of sorbed gas, the bed is desorbed and/or purged to remove contained nitrogen before being returned to on stream operation. The adsorption-desorption cycles rely chiefly on swing in pressure level of these steps in the operating cycle. Also, to maintain continuity of operation a number of such adsorbent beds are operated in parallel, so that while one bed is operating on the adsorption step of the cycle, companion beds are at various stages of regeneration.
Illustrative of well-known prior art systems proposed for separation of components of air by selective adsorption is that disclosed in U.S. Pat. No. 2,944,627.
Numerous asserted improvements or modifications of the system described in the '627 patent are disclosed in the patent art. Among these are various sequences employing a vacuum desorption step for withdrawl of sorbed nitrogen from the adsorbent bed, as in U.S. Pat. Nos. 3,155,468; 3,164,454; 3,473,296 and 3,797,201. The listed patents suggest the possible use of a single column for carrying out the disclosed process.
It has also been proposed in a number of patents to remove water and CO.sub.2 from the air feed prior to subjecting the same to selective adsorption for N.sub.2 --0.sub.2 separation. U.S. Pat. No. 3,533,221 is illustrative of these. Preceding a nitrogen adsorption column the system employs two separate parallel alternately operating beds of sorbent for removal of water and CO.sub.2. The moisture and CO.sub.2 -laden beds are regenerated thermally; one of these beds being heated and then cooled under purge while the other is on stream. The nitrogen laden column is desorbed by vacuum, the desorbed gas as it is being withdrawn passing through the water-CO.sub.2 bed then under regeneration. Such operation sequence entails a complex arrangement of beds, valves and switching devices.
Pretreatment of the feed air in a separate bed to remove water and CO.sub.2 is also disclosed in U.S. Pat. No. 3,796,022. The patent is more particularly concerned with operations wherein the desired recovered product is that obtained by desorption. Thus, in the case of air separation, wherein nitrogen is preferentially retained by the adsorbent bed, an elution step is had wherein the column is purged with nitrogen product gas in air feed direction, the purged product being withdrawn with aid of vacuum, and discharged to the atmosphere. In this manner, it is stated, the oxygen co-adsorbed with the nitrogen is eliminated, enabling recovery of high purity nitrogen by vacuum desorption applied at the opposite end of the column.
U.S. Pat. No. 3,280,536 describes an air separation system to increase the oxygen content of an enclosure, such as an entire room, to the range of 30 to 50% oxygen. While other known adsorbents for nitrogen are noted in the patent, including 13X zeolite, the stated preference is for 13X zeolite which has been partially substituted with strontium. The operating cycle is according to the principle of the aforementioned U.S. Pat. No. 2,944,627, and several hours are required to raise the room concentration to the 30% oxygen level. There is no indication in the patent of how the system could be employed to produce a product stream of high oxygen content, in the order of 90% O.sub.2. The disclosed operating cycle includes an initial step of pressurizing the column to the super atmospheric working pressure with feed air. Under these conditions it would be difficult, if not impossible, to obtain high oxygen recovery at 90% purity.
In recent years there has been a significant increase in the demand for small scale oxygen generators for medical uses, to replace conventional gas cylinders.